1. Field of the Invention
This invention relates to an improvement in a permanent magnet for an electronic lens used for e.g., electron beam control of a cathode-ray tube for a projector.
2. Prior Art
For a device of this kind, there has been already proposed a first prior art shown in FIG. 5 as the side cross sectional view.
As shown in FIG. 5, a ring-shaped permanent magnet 1 as an electronic lens is fitted on a neck portion 8 of a cathode-ray tube (glass bulb) so as to render the lens effect to an electron beam emitted toward the front side of the electron gun, and to allow an image on the fluorescent surface to have a small spherical aberration and produce no halation.
This permanent magnet 1 is magnetized as shown in a Z-axis direction, i.e., in an axial direction along which an electron beam travels. Onto both magnetic pole end surfaces, ring-shaped flat plate like magnetic pole plates 2a, 2b of magnetic body are bonded. These plates are circumferentially fitted in a manner that the inner circumferential surface of the ring-shaped permanent magnet 1 is slightly spaced from the outer circumferential surface of the neck portion 8. An electron beam 9 emitted from the cathode 4 toward the anode 7 travels while adjusted by first and second grids 5 and 6 on the way to pass through an inner hole of the electron lens (permanent magnet 1). As a result, the electron beam 9 is focused to reach the fluorescent surface (not shown).
FIG. 6 shows the construction of the first prior art of the electron lens wherein FIG. 6(a) is a side cross sectional view and FIG. 6(b) is a front view. Further, FIG. 4 is an actual measurement characteristic curve diagram of the width of the magnetic density curve in a direction of the electron beam showing the length in a direction of the electron beam of the permanent magnet and the half-value of the maximum magnetic flux density in an electron lens comprised of a permanent magnet alone.
A second prior art is disclosed in the Japanese patent application laid open No. 211940/86.
In all drawings, the same reference numerals denote the same or corresponding members, respectively.
It is disclosed in the second prior art (shown in FIG. 8 as the side cross sectional view) that a permanent magnet for an electronic lens comprises at least two ring-shaped permanent magnets 1a, 1b, . . . which are individually magnetized and are connected in the same magnetic pole direction, and the half-value width of the magnetic flux density distribution on the Z-axis of the permanent magnets 1a, 1b . . . have 80 to 200% of the inner diameters of the permanent magnets. In this example, reference numerals 2.sub.1a, 2.sub.1b, 2.sub.2a and 2.sub.2b all denote magnetic pole pieces, respectively.
However, in the case of the structure of the electronic lens shown in the above-mentioned first prior art, as shown in FIG. 7, the magnetic flux density distribution diagram in the Z-axis direction from the magnetic pole piece 2a to the magnetic pole piece 2b is represented by the magnetic flux density curve 15.
When an attempt is made to set the width in the Z-axis direction indicating the half-value 15b of the value 15a at which the magnetic flux density is maximized (which is referred to as "half-value width") to a fairly good value, e.g., about 26 mm, the length (thickness) in the Z-axis direction of the permanent magnet of FIG. 6(a) must be considerably elongated. For this reason, the permanent magnet is difficult to manufacture, and becomes expensive.
On the other hand, it is seemingly true that the second prior art has eliminated the drawbacks with the first prior art.
FIG. 9 is a magnetic flux density distribution diagram in the second prior art of FIG. 8 wherein the magnetic flux density curve 17 shows the distribution in the Z-axis direction from the magnetic pole piece 2.sub.1a to the magnetic pole piece 2.sub.2b, and 17a and 17b represent the magnetic flux density maximum value and the half-value of the maximum value, respectively. Plotting in FIG. 8 is made in comparison with the first prior art.
However, the second prior art is only considered as means adapted so that the permanent magnet of the first prior art is divided into two sections to manufacture them as the permanent magnets 1a and 1b, respectively, to connect these magnets in the Z-axis direction. Although the manufacturing cost is somewhat reduced in the case of the second prior art, there is not so positively appraised improvement from a viewpoint of the use requirement of a large quantity of permanent magnets of the expensive member, its operation or effect, and the advantage indicating to what degree the second prior art has been advanced as compared to the first prior art.
In the case of an electronic lens of this kind, a control scheme is employed to allow a direct current to flow in the excitation coil to superimpose a magnetic flux in the same direction as the direction of a magnetic field produced by the permanent magnet to control the value of the direct current, thus to adjust the strength of the magnetic field.
FIG. 11 shows the construction of a permanent magnet to which means according to this invention provided with a yoke 3 of a ferromagnetic body is applied, wherein a lead wire 17 for supplying a current to the excitation coil is connected to a coil 15 wound onto a bobbin 14 via a take-out hole 16 bored or opened in the magnetic pole piece 2a.
For this reason, it is required to ordinarily provide a take-out hole 6 on one side of the magnetic pole piece 2a or 2b.
For ordinary magnetic pole piece or yoke, soft iron such that the content of carbon is less than 0.3% is used.
However, the provision of the take-out hole 16 for taking out the lead wire 17 to the outside in the magnetic pole piece 2a allows the physical condition of the magnetic pole piece 2a to be uneven, resulting in disturbed or inhomogeneous strength distribution of the magnetic field based on the magnetic flux 9.
In addition, because soft iron such that the content of carbon is less than 0.3% is used as the magnetic pole piece or the yoke, where a high frequency magnetic field is produced in the vicinity thereof, an undesirable elevation of temperature due to eddy current loss would take place.